SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21551, 21552, 21553, 21554, 21555, 21556, 
    21557, 21558, 21559, 21560, 21561, 
    21562, 21563, 21564, 21565, 21566
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00070

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (21551,21552,21553,21554,21555,21556,21557,21558,21559,21560,21561,21562,21563,21564,21565,21566) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21551 bush<p>Part number 54553T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21552 BOLT<p>Part number 54561T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21553 Shock absorber assembly-front, left<p>Part number 54605T1450</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21554 spring-front<p>Part number 54630T1210</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21555 Shock Absorber Assembly-Front, Right<p>Part number 54606T1450</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21556 spring-rear<p>Part number 55330T1500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21557 BOLT<p>Part number 577623E000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21558 insulator<p>Part number 21772B1200</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21559 bush<p>Part number 54553T1200</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21560 plate<p>Part number 55223D2100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21561 Protector, left<p>Part number 55295T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21562 Protector, right<p>Part number 55296T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21563 Crossmember-rear<p>Part number 55405T1111</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21564 plug<p>Part number 55429T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21565 BOLT<p>Part number 1140612406K</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21566 Washer-Plain<p>Part number 51788J9100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>